The late game has an unparalleled ability for you to sustain a constant world war - your empire is fairly developed, so you can devote to full time military production; you have fascism, which is enormously powerful with the +50%

towards units and +5

to all units. And you have the almighty airforce. Unlike civ5, air units don't take damage when attacking
unless they come up against something with anti air strength. So you can just keep raining bombs from the sky.
The AI really doesn't have the ability to commit to unending unit production like you do, so you will eventually win wars. There's nothing about modern combat that is any slower than earlier eras - being able to afford an airforce and/or nuclear weapons and having some tanks or mechanized infantry will help you sprint through an empire in no time. In the early game, you have to constantly move you army and siege units around. But in the late game, you can take multiple cities from one airbase and blitz a few tanks around to capture the 1hp cities.
Alternatively, for those with uranium, you can just nuke the core of enemy empires off the map and stroll in to take the capital afterwords. Alternatively, if you get to the end of the tech tree, the giant death robot is completely OP for what it costs and can bust through cities in 1 to 2 turns.
Always build Armies and Armadas - they have +17

strength, or +100%, they have lower maintenance than 3 separate units, and they cost the same resource maintenance. Keep in mind that +100% means they deal more damage
and take less so their actually effectiveness is very high. And, you get a

discount for building them in a city with a military academy.
Combining those factors together, you should be able to dish out an unending stream of units to multiple fronts and crush most empires in almost no time at all. You only need to take the capitals, so seizing the capital and maybe a few big core cities and just settling peace is enough to then move on. You can also look for empires with vulnerable capitals near the border or on the coast, and plan to snipe them around the same time to finish the game off, since once you hold all 8 you win.